Identification of novel central valley Trichoderma isolates for biological control of pierce’s disease in California

Authors: Wallis, Christopher

Submitted to: Symposium Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 4/23/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: Pierce’s disease is a costly disease for grapevine growers to manage. Thus, this study examined whether endophytic Trichoderma sp. isolated from Californian grapevines could prevent or ameliorate Pierce’s disease. Obtained isolates included species with known biological control activity (Trichoderma harzianum and Trichoderma asperellum), species without known activity (Trichoderma capillare and Trichoderma saturnisporopsis), and two novel species. Greenhouse studies observed that prior inoculation by two Trichoderma isolates significantly reduced titers of Xylella fastidiosa, the causal agent of Pierce’s disease, albeit only in one of two experimental trials. Two isolates also reduced Pierce’s disease symptoms, albeit not statistically. Thus, these results demonstrate that the tested Trichoderma isolates likely could not be relied upon to limit Xylella fastidiosa infections or prevent Pierce’s disease. However, inoculation of Trichoderma by these isolates can provide other benefits to grapevine health, as they were observed to reduce or eliminate fungal pathogen infections. They also were able to survive the hot, dry summer conditions in Central California. Therefore, findings from this study could result in novel, biological control products available for growers to improve grapevine health, albeit the use of these isolates to manage Pierce’s disease would likely need to be coupled with other management strategies.

Technical Abstract: Pierce’s disease of grapevine, caused by the bacterium Xylella fastidiosa ssp. fastidiosa, is a serious and fatal disease where endemic. Current management involves the use of pesticides to control spread by sharpshooter vectors, but long-term, sustainable and ecologically friendly options are desired. Thus, isolates of the fungal biological control genera Trichoderma were obtained from Californian vineyards to screen whether inoculation of these into grapevines could prevent Pierce’s disease and reduce Xylella titers. These previously were observed to reduce fungal pathogen infections, and being sourced from Californian vineyards should allow fewer restrictions on deployment and the pre-adaption to the local hot, dry summers. Indeed, these isolates were capable of being reacquired from field-inoculated vines after an entire summer. However, studies observed that vines inoculated with one of the eight Trichoderma isolates did not have significantly reduced Pierce’s disease progression. Furthermore, reductions in Xylella titers were inconsistent, although a couple of isolates may have minor inhibitory effects on bacterial populations. In addition to bioassays, characterizations of the eight isolates were performed, and this revealed the collection included two isolates each of Trichoderma harzianum and Trichoderma capillare, one isolate each of Trichoderma asperellum and Trichoderma saturnisporopsis, and one isolate each of two putative novel Trichoderma species. Despite the findings that these Trichoderma isolates may not be consistently effective options to combat Pierce’s disease, they still could improve grapevine health as they can combat other pathogens and increase tolerance to different stresses.